Method for spray-cooling a tunable semiconductor device

ABSTRACT

The method includes providing a substrate (18) having a semiconductor die (12) thereon, the semiconductor die having a major surface (23); disposing an input region (24) an active region (32) and a conductive region (34) on the major surface, the conductive region electrically coupling the input region and the active region; providing an inductive metal region (42) in communication with the input region, the inductive metal region sized to allow real-time iterative tuning of the electronic device; and receiving a fluid by a nozzle (60), the nozzle atomizing the fluid and discharging the fluid onto the major surface.

This is a divisional of application Ser. No. 08/729,130, filed on Oct.11, 1996 now U.S. Pat. No. 5,777,384.

FIELD OF THE INVENTION

This invention relates generally to electronics, and, more particularly,to an electronic device and a method for spray-cooling an electronicdevice.

BACKGROUND OF THE INVENTION

Often, electronic devices are cooled by natural or forced air convectionwhich, because of the relatively poor thermal capacitance and heattransfer coefficients of air, requires moving large volumes of air pastthe devices or past heavy heat sinks attached to the devices. When largeheat sinks are utilized, the overall weight and size of electronicequipment may be increased.

Evaporative spray cooling features the spraying of atomized fluiddroplets directly onto a surface of a heat source such as an electronicdevice. When the fluid droplets impinge upon the device's surface, athin film of liquid coats the device, and heat is removed primarily byevaporation of the fluid from the device's surface.

Although evaporative spray cooling is a preferred method of heat removalin many electronics applications, spray-cooling some electronic devicesmay not be desirable or practical. For example, the wire interconnectsof typical high-power microwave and radio frequency transistors whichare utilized to form highly critical inductive portions of frequencydependent impedance matching structures essential to optimal deviceperformance as well as interconnection between various othersemiconductor dice, metal oxide semiconductor capacitors, packagefeatures such as leads, and other components associated with thetransistors present significant practical application, performance,assembly handling, and manufacturing liabilities when associated withthe implementation of spray cooling technology.

Wires are fragile. In applications such as microwave and radio frequencypower transistors where minor perturbation of the wire structure willresult in device failure either through detuning of the matchingstructure or physical shorting of the wires together, the wires may nottolerate being sprayed by a fluid. Microwave aid radio frequencytransistors and assembly modules with chip and wire matching are alsoextremely sensitive to variations in shape and spatial positioning ofthe wires. Misalignment and bending of the wires may adversely affectdevice tuning thereby altering device performance. In addition, wiresare often protected from physical damage by a device cap. The devicecap, however, may preclude effective spray-cooling of the transistorbecause it does not allow for the spraying of fluid directly onto thesurface of semiconductor dice.

Moreover, wire-bonded wires may preclude automated iterative tuning ofan electronic device, which is desirable to minimize performancevariation and improve product yield.

There is therefore a need for an electronic device which toleratesdirect spraying of a semiconductor die within the device, whicheliminates the need for wire bonding, and which may be iteratively tunedduring operation of the device to reduce performance variation.

SUMMARY OF THE INTENTION

According to an aspect of the present invention, the foregoing needs areaddressed by a method for spray-cooling an electronic device whichincludes providing a substrate having a semiconductor die thereon, thesemiconductor die having a major surface; disposing an input region, anactive region and a conductive region on the major surface, theconductive region and electrically coupling, the input region and theactive region; providing an inductive metal region in communication withthe input region, the inductive metal region sized to allow real-timeiterative tuning of the electronic device; and receiving a fluid by anozzle, the nozzle atomizing the fluid and discharging the fluid ontothe major surface.

Advantages of the present invention will become readily apparent tothose skilled in the art from the following description of the preferredembodiment(s) of the invention which have been shown and described byway of illustration. As will be realized, the invention is capable ofother and different embodiments, and its details are capable ofmodifications in various respects. Accordingly, the drawings anddescription are to be regarded as illustrative in nature, and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an electronic device according to one embodimentof the present invention.

FIG. 2 is a front view along line 2--2 of the electronic device shown inFIG. 1, mounted to a substrate in accordance with one embodiment of thepresent invention.

FIG. 3 is a front view of the electronic device depicted in FIGS. 1 and2, mounted to a substrate in accordance with another embodiment of thepresent invention, further depicting a spray-cooling system having aclosed-loop fluid flow.

FIG. 4 is a top view of an electronic device according to a furtherembodiment of the present invention.

FIG. 5 is a front view along line 5--5 of the electronic device depictedin FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, wherein like numerals designate likecomponents, FIG. 1 is a top view of an electronic device 10 according toone embodiment of the present invention. As shown, electronic device 10is a laterally-diffused metal oxide semiconductor radio frequency powertransistor. It is contemplated, however, that electronic device 10 maybe another type of device, such as a an integrated circuit or a bipolaror field effect transistor. Device 10 includes a transistorsemiconductor die 12 and matching metal oxide semiconductor (MOS)capacitor semiconductor dice 14, 16, disposed on a substrate 18.Substrate 18 may be ceramic or another suitable material. Transistorsemiconductor die 12 is referred to herein as transistor 12, capacitorsemiconductor die 14 is referred to as input capacitor 14 and capacitorsemiconductor die 16 is referred to as output capacitor 16. In addition,electronic device 10 may be configured in a variety of ways, forexample, vertically integrated.

Transistor 12 has a major surface 23. An input region 24, which ispreferably a metallized polyamide, having a number of input devicecontacts 26 associated therewith, is disposed on major surface 23. Anoutput region 28, which is preferably a metallized polyamide, having anumber of output device contacts 30 associated therewith, is alsodisposed on major surface 23. Device contacts 26, 30, which arepreferably disposed on die 12, may be a metal such as aluminum or goldwith a suitable solder overcoat, may be soldered or otherwise secured toinput region 24 and output region 28, respectively.

Active regions 32 of transistor die 12, which may include a number ofcells, each cell including arrays of transistors such aslaterally-diffused metal oxide semiconductor (LDMOS) transistors, areelectrically coupled to input region 24 (via input device contacts 26)and output device region 28 (via output device contacts 30) byconductive region 34, which is co-planar with input region 24 and outputregion 28. Active region 32 may be ballasted with resistive elements,for example, to balance a flow of current through each of the activecells associated with active region 32. Ballasting regions may beimplanted in die 12 via an ion implantation process according towell-known methods, may be diffused into die 12 or alternatively may bedeposited onto die 12 using vapor deposition, for example.

Conductive region 34 includes an input edge contact 36 and an outputedge contact 38, and also includes active region contacts 41, or feedstructures. Faraday shielding structures (not shown) may be used inconjunction with conductive regions 34 to mitigate capacitance effects.It is also contemplated that active region contacts 41 may connect toconductive regions 34 from the center of die 12.

Like transistor 12, input capacitor 14 has an input region 80 and anoutput region 82. Output capacitor 16, which is optional, also has aninput region 84 and an output region 86. Input regions 80,82 and outputregions 84, 86 are preferably composed of a metal such as gold.

Electrically reactive metal regions 42, such as inductive metal regions,which may be copper or another suitable metal, are disposed on substrate18 and are in communication with input region 24 and output region 28 oftransistor 12 and with input regions 80, 84 of input and outputcapacitors 14, 16, respectively. Inductive metal regions 42 are sized toallow real-time iterative tuning of transistor device 10. For example,inductive metal regions 42 may be etched in a well-known manner using alaser.

FIG. 2 is a front view along line 2--2 of the electronic device shown inFIG. 1, mounted to substrate 18 in accordance with one embodiment of thepresent invention. Transistor 12 is tied to ground layer 61 via agrounded die bond pad 29 and an array of vias 20. Flex circuitinterconnects 44, which may be made of a metallized polyimide materialor another suitable material, are positioned over input device contacts26 or over output device contacts 30 of transistor 12, or over inputregions 80, 84 or output regions 82, 86 of capacitors 14, 16,respectively, and are also in contact with substrate 18. Flex circuitinterconnects 44 may be secured to substrate 18 using a well-knownmethod such as a solder bump reflow process.

Interconnects 44 are preferably compliant to absorb strain introduced bydiffering coefficients of thermal expansion of dice 12, 14 and 16 andsubstrate 18, which is preferably ceramic but may be another suitablematerial such as glass-filled epoxy, teflon or plastic. Suitableinterconnects 44 are commonly available from a variety of sources. Asshown in FIG. 2, interconnects 44 are "S"-shaped to accommodate theheight difference between dice 12, 14 and 16 and substrate 18.

FIG. 3 is a front view of electronic device 10, mounted to substrate 18in accordance with another embodiment of the present invention. Asshown, an additional substrate layer 19, made of ceramic or anothersuitable material, is in communication with substrate 18, havingcavities 21 therein. Dice 12, 14 and 16 are disposed within cavities 21.Interconnects 44, which are preferably flat, secure dice 12, 14 and 16to substrate 19. A ground metal layer 61 may be disposed betweensubstrate 19 and substrate 18.

FIG. 3 further depicts a spray-cooling system having a closed-loop fluidflow, suitable for use with electronic device 10. One or more nozzles 60are preferably disposed in nozzle housing 40, which may be anygeometrical shape such as rectangular or cylindrical, and is preferablymetal, but may be constructed from another material such as plastic. Twonozzles may positioned over transistor 12, while a single nozzle may bedisposed over each capacitor 14, 16.

Nozzles 60 are preferably miniature atomizers such as simplexpressure-swirl atomizers, and may be made of any suitable material. Anexample of a suitable material is a metal such as stainless steel orbrass. Simplex pressure-swirl atomizers are described in detail. in U.S.Pat. No. 5,220,804 to Tilton et al., incorporated herein by reference,and are commercially available from Isothermal Systems Research, Inc.,located in Colton, Wash.

A fluid pump 50, which is connected via tube 52 to fluid inlet port 46,supplies a coolant fluid to a fluid supply manifold 70, suitable fluidsupply manifolds being well-known, which may be used to deliver thefluid to nozzles 60. Nozzles 60 atomize the coolant and discharge anatomized fluid 70 directly onto dice 12, 14 and 16. When atomized fluid70 impinges upon the surfaces of dice 12, 14 and 16, a thin liquid filmcoats the dice, and heat is removed primarily by evaporation of fluid 70from the dice. Excess fluid may be collected and removed by fluid returnmanifold 72 and outlet port 48.

The coolant fluid may be any dielectric coolant, such coolants beingwell-known and widely available. One example of a suitable coolant is3M's Fluorinert™ dielectric fluid, available from 3M, order numberFC-72. Another perfluorocarbon fluid similar to 3M's Fluorinert™dielectric fluid is available from Ausimont Galden®.

A condenser 53, connected to pump 50 by tube 54 and to fluid outlet port48 by tube 56, receives fluid from fluid outlet port 48. Condenser 53rejects heat from the fluid, returning it to primarily a liquid phase.Fan 58 may be used to extend the cooling capacity of condenser 53.Cooled fluid is supplied from condenser 53 to pump 50. Thus, aclosed-loop flow of coolant is formed. It will be appreciated that atany given point the coolant may be a vapor, a liquid or a vapor andliquid mixture.

It is contemplated that any conventional means for providing flow of acoolant may be used in conjunction with the described embodiments of thepresent invention, and that more than one housing 40 may be connected toa single source of coolant or that one or more sources of coolant may beconnected to a single housing 40, for example, for redundancy purposes.

Sizes of fluid pump 50, condenser 53 and fan 58 should be selected basedon heat removal and flow rate requirements. For example, a typicalclosed-loop fluid flow is 500 to 1000 milliliters per minute for 500 to1000 Watts of heat dissipation. Pump and condenser assemblies in varioussizes are available from Isothermal Systems Research, Inc., andacceptable tubing and fittings may be obtained from Cole-Parmer inVernon Hills, Ill.

An electronic device or a group of electronic devices having a powerdensity of up to 300 Watts per square centimeter is effectively cooledusing the disclosed apparatus. The removal of heat directly fromindividual dice, or from groups of dice, rather than from a protectivedie shield or heat sink, helps to reduce operating temperatures of thedice, increasing reliability through reduction of thermal variation andassociated thermal stresses.

Housing 40 may be placed extremely close to the surface of electronicdevice 10 because spacing is not governed by air volume requirements.Thus, packaging size for the electronic device may be reduced. Inaddition, unlike air cooling, which is most effective when heat isspread over a large area, for example, over a large heat sink,spray-cooling encourages heat concentration, another factor contributingto reduced packaging volume and weight.

Furthermore, electronic device 10 described herein eliminates the needfor fragile wire bond wires completely and replaces them with a robustalternative. The replacement of wire bond wires with fixed-positioncontact structures such as flexible interconnects allows for automatediterative tuning, minimizes performance variation and improves productyield. Thus, embodiments of the present invention are desirable forcooling an electronic component during the testing and tuningprocess--the electronic module may be tested, laser-tuned andspray-cooled simultaneously.

FIG. 4 is a top view of an electronic device according to a furtherembodiment of the present invention, and FIG. 5 is a front view alongline 5--5 of the electronic device depicted in FIG. 4. Electronicdevices 10 are disposed on both sides of substrate 18 as described inconnection with FIG. 3. Rather than securing devices 10 to substrates 19using interconnects 44, however, further substrate layers 90 havingfluid distributing channels 72 therein are coupled to substrates 19.Dice 12, 14 and 16 are disposed within fluid distributing channels 72.Inductive metal regions 42 are located on substrate layers 90.Communication between inductive metal regions 42 and dice 12, 14 and 16may be effected by vias 20. The electronic device depicted in FIGS. 4and 5 is a compact, three-dimensional transistor matrix whichaccommodates spray-cooling as described herein through fluiddistributing channels 72 and is physically robust.

It is contemplated that any form of cooling may be utilized with theelectronic device of the present invention, for example, air coolingand/or heat sinks and other types of cooling.

It is also contemplated that wherever sealing and/or fastening may berequired, numerous methods and materials may be used such as soldering,adhesives or fired ceramic.

It will be apparent that other and further forms of the invention may bedevised without departing from the spirit and scope of the appendedclaims and their equivalents, and it will be understood that thisinvention is not to be limited in any manner to the specific embodimentsdescribed above, but will only be governed by the following claims andtheir equivalents.

We claim:
 1. A method for spray-cooling an electronic device, comprisingthe steps of:providing a substrate having a semiconductor die thereon,the semiconductor die having a major surface; disposing an input region,an active region and a conductive region on the major surface, theconductive region electrically coupling, the input region and the activeregion; providing an inductive metal region in communication with theinput region, the inductive metal region sized to allow real-timeiterative tuning of the electronic device; and receiving a fluid by anozzle, the nozzle atomizing the fluid and discharging the fluid ontothe major surface.
 2. The method according to claim 1, furthercomprising:providing the fluid to the nozzle via a fluid inlet port; andremoving the fluid from the major surface via a fluid outlet port. 3.The method according to claim 2, further comprising:receiving the fluidfrom the fluid outlet port by a condenser; and supplying the fluid bythe condenser to a fluid pump, the fluid pump in communication with thefluid inlet port,